Coherent light can theoretically travel an indefinite distance in free space, as there are no intrinsic limitations to its propagation. In practice, however, several factors can affect the distance over which coherent light can travel effectively.
The primary factor is the divergence of the light beam, which refers to the spreading out of the beam as it propagates. Divergence occurs due to diffraction effects and the finite size of the beam aperture. As the beam diverges, its intensity decreases, making it harder to detect or use over long distances. The rate of divergence depends on the wavelength and the size of the beam.
Another factor is atmospheric absorption and scattering. As coherent light passes through the atmosphere, it interacts with various particles and molecules, leading to absorption and scattering. These interactions can cause attenuation and degradation of the light beam, reducing its intensity and coherence. The specific effects depend on the wavelength of the light and the composition and condition of the atmosphere.
In practical scenarios, the usable range of coherent light depends on the application and the available technologies. For example, in telecommunications, coherent optical fibers are used to transmit signals over long distances, typically thousands of kilometers, with the help of amplifiers to compensate for signal attenuation. In free-space optical communications, where light is transmitted through the atmosphere, the usable range is generally limited to several kilometers due to atmospheric effects.
To summarize, while coherent light can theoretically propagate indefinitely in free space, practical limitations such as divergence, atmospheric absorption, and scattering impose constraints on its effective range in real-world applications.